This invention relates to a circuit breaker with an improved magnetic trip assembly and more particularly to a magnetic trip assembly including a plate mounted to the armature and disposed between the armature and the stationary magnetic structure of the magnetic trip assembly to adjust the attraction force between the armature and the stationary magnetic structure.
Circuit breakers provide protection for electrical systems from electrical fault conditions such as current overloads and short circuits. Typically, circuit breakers include a spring powered operating mechanism which opens electrical contacts to interrupt the current through the conductors on an electrical system in response to abnormal currents. The operating mechanism is unlatched by a trip bar which in turn is operated by a trip mechanism associated with each phase of the electrical system. The trip mechanism can include a magnetic trip device comprising a fixed magnetic structure energized by the current flowing through the conductor, and a movable armature which is attracted toward the stationary magnetic structure to operate the trip bar. The trip bar in turn unlatches the operating mechanism to open the electrical contacts in each phase of the electrical system. The movable armature is biased away from the stationary magnetic structure by a spring thereby forming a gap between the armature and the stationary magnetic structure in the absence of an abnormal current.
Usually, means are provided for adjusting the level of current at which the magnetic trip device actuates the operating mechanism. Such adjustments can be made by varying the spring bias applied to the armature and/or mechanically adjusting the gap such as by varying the position of a threaded screw or cam against which the spring biases the armature. These adjustments permit fine tuning of the circuit breaker to assure that it will operate at the desired level of fault current. They can also be used to provide a range of settings at which the circuit breaker will trip. For instance, a circuit breaker may be rated to provide a range of trip settings between a low of say 500 amperes and a high of 1000 amperes.
During manufacturing of the circuit breakers, tests are performed to determine if the circuit breaker is performing properly at the rated trip settings. Using the example set forth above, the circuit breaker is tested at the low trip setting to see if the trip amps fall into an acceptable range, which is approximately plus or minus 10% (450 amps to 550 amps). The same testing is done for the high trip setting, with that range being also plus or minus 10% (900 amps to 1100 amps). Experience has shown that a substantial percentage of the circuit breakers fail one or both of these tests, leading to scrapping of the entire unit. The causes of these failures include discrepancies in the magnetic field caused by differences in materials used for the magnetic trip assembly, size differences (due to manufacturing tolerances) in the parts used in assembling the circuit breaker and incorrect or imprecise assembly of the various parts used for making the circuit breaker.
Thus, what is needed is a circuit breaker that includes a magnetic trip assembly having an adjustment means that can be used to compensate for the above-mentioned inevitable imperfections in the manufacturing and assembling processes.